The invention relates to a particularly economic process for the production of nitric acid with an approx. content of 50 to 76%, said process using the combustion of ammonia under pressure with the aid of an oxygen-bearing gas, in particular air, thereby simultaneously exploiting the reaction heat for the recovery of useful energy made available as steam or electric power.
Nitric acid with a concentration of 50 to 76 percent is normally produced in plants using the Ostwald process as described, for instance, in the paper “Inorganic oxygen compounds” published by Mundo/Weber, Carl Hanser Publishing House in Munich and Vienna, 1982, a special print extracted from “Winnacker/Küchler” Chemische Technologie, volume 2, Inorganic Technology I, 4th edition, and in WO 01/68520 A 1.
According to said technology there are a mono-pressure and a dual pressure process. The monopressure processes first provide for a compression of the air to medium pressure of about 5 bar or to a high pressure of about 8 bar, pressurised ammonia being admixed and the mixture then being catalytically burnt. Upon cooling, the nitric oxide thus formed comes into contact with compressed secondary air and is absorbed in an aqueous solution so that nitric acid is obtained. The residual gas hereinafter referred to as tail gas is purified and released into the atmosphere.
Downstream of the catalytic combustion and the subsequent cooling, the dual pressure process provides for a further compression step in order to run the absorption at a pressure higher than that of the combustion.
The single or multi-stage compression of the input gas, normally air, and the expansion of the tail gas usually take place in turbo-compressors and turbo-expanders, all of which are arranged on a common gear shaft an thus interconnected with each other, the different speeds of the machines being compensated by means of drive gears. As the potential energy produced by the gas expansion is normally insufficient for operating the compression unit of the input gas, it is necessary to provide additional drive energy.
The state-of-the-art technique provides for this additional drive energy by way of an energy-optimised steam turbine operated with the aid of a portion of the steam obtained by cooling of the gas rich in nitric oxide in steam generators arranged downstream of the catalytic combustion step. The said steam turbine is also exploited for the process start-up and persons skilled in the art consider this unit to be indispensable.
A disadvantage, however, is the steam consumption of the turbine which accounts for one fifth to one third of the overall steam production. This particularly applies to the said configuration if the steam thus produced could be used for other purposes in the plant, e.g. for nitric acid concentration, electric power production or heating etc. provided the plant for nitric acid production is part of a larger complex. But if a steam turbine is not provided as auxiliary drive it is necessary to boost the residual gas expander capacity in such a manner that its output equals the power input of the compressors. Using this technology in accordance with the conventional technique will produce problems that can hardly be overcome.
An aim of the invention is to redesign the nitric acid plant in such a manner that more useful energy becomes available in the form of process steam or electric power for export to external power consumers. The aim also includes that the standard operation of a steam turbine should be omitted and that the steam quantity thus saved be exploited for other purposes or that the said standard operation of a steam turbine should essentially be devoted to power generation instead of compressor drive. Moreover, the aim also encompasses a configuration that has no steam turbine at all yet providing an economy.